Android TabActivity中onKeyDown无法响应的解决方法

Android中某个类继承Activity的子类TabActivity时，重载Activity中的public boolean onKeyDown(int keyCode, KeyEvent event)方法时，点击按键时并得不到响应，解决的方法是：

重载public boolean dispatchKeyEvent(KeyEvent event)方法，其示例代码如下：

private long exitTime = 0L;

@Override
public boolean dispatchKeyEvent(KeyEvent event) {
if (event.getKeyCode() == KeyEvent.KEYCODE_BACK) {
if (event.getAction() == KeyEvent.ACTION_DOWN
&& event.getRepeatCount() == 0) {
if (System.currentTimeMillis() - exitTime > 2000) {
Toast.makeText(this, "再按一次退出", Toast.LENGTH_LONG).show();
exitTime = System.currentTimeMillis();
} else {
this.finish();
System.exit(0);
}
return true;
}
}
return super.dispatchKeyEvent(event);
}

这是退出的例子代码